I'm simulating an amplifier and want to know if it is possible to define its SNR. A simulation in LTSpice shows that the circuit generates 42nV/Hz, but this value is independent from the input/output voltage. Obviously the signal-to-noise ratio will vary depending on the output signal. Is there maybe a standardized output voltage in relation to which SNR is usually expressed? I see that audio equipment, like preamplifiers and audio recorders, always indicate their SNR in dB. But in relation to what output voltage?
Also, output voltage will depend on input voltage and the overall noise of a circuit should vary according to it, I guess. ie. thermal noise from resistors should change with input voltage. Does Spice not take this into account? Probably I'm falling into misconceptions or I am not simulating this properly.
Spice directive: .noise V(n001) V1 dec 10 30 15k